It is well known that DC to DC converters can be made to step down, that is BUCK, or step up, that is BOOST, a DC input voltage to a different DC output voltage. The DC to DC converter may comprise an inductor in association with two or more transistors such that the current in the inductor can be built up such that energy is stored in it by virtue of its magnetic field, and then that energy can be discharged from the inductor in order to charge a storage capacitor at the output of the DC to DC converter.
Where a DC to DC converter operates with, for example, a battery as the input voltage source then the battery voltage may change as the battery discharges. Thus a converter may initially be required to BUCK the input voltage, but as the battery discharges it may move into a BOOST mode. It follows that at some time the input voltage may be close to the output voltage. This regime can be difficult to control.
It should be noted that such inductor based DC to DC converters inherently require a ripple current to occur in the inductor otherwise the converter loses its ability to regulate its output voltage in response to changes of load current. It becomes important to be able to control this ripple.